That's true, but on the other hand their assertion is also true - terrorists (both political and paramilitary) do attempt to exploit the infrastructure of an open society, and government (qua public agency) does have a legitimate interest in frustrating some kinds of criminal traffic.

You try running a government and explaining to people that the price of liberty is putting up with a certain level of terrorism and child porn every year. You won't stay in power long if you do. (Mind, I'm not endorsing this new law, which I think is awful. I'm just taking issue with your objection to the political reasoning, and saying that it's self-defeating.)